What are my options for cooling a greenhouse?

 Q: What are my options for cooling a greenhouse?

In many parts of the United States, cooling a greenhouse is a greater problem than heating it. When ventilation will not control temperature, shading can be used. By reflecting the sun’s rays or blocking them so they can’t get into the house, a significant portion of the heat load is removed. This can reduce electricity use associated with fan ventilation systems.

The heat load is created by the infrared wavelengths of the radiant energy from the sun. Plants are photosynthetically active in the blue to red range (380–700 nanometers) of the total light spectrum. Wavelengths greater than 770 nanometers result in an increase in temperature. This can affect plant growth and potentially cause plant injury. It’s the same as standing in the sun or putting your hand next to an incandescent light bulb. You are feeling the far-red radiation.

Reducing the heat load in the greenhouse can be done in a number of ways. Ventilation can cool a greenhouse to near the ambient outdoor temperature. Further cooling requires introducing mist or fog into the air to absorb some additional heat. Fan-and-pad or fog systems are available for this. These can require considerable energy for fan or high pressure pump operation.

Shading is one method of controlling greenhouse temperatures during warm weather. It can supplement fan or louver ventilation and evaporative cooling. It also helps to reduce plant leaf temperature reducing burning.

Shading is best done outside the greenhouse over the glazing, but can also be done inside with a screen system. Shading can result in lower electricity bills if fans are used.

Small Budget Solution
Shading compounds are materials that are brushed, rolled or sprayed onto the glazing. They are applied in the spring and if the correct material is used, they usually wear off before the first frost occurs. These materials can be difficult to apply uniformly. Some growers add additional layers of shading as the sun’s intensity increases during the summer. Some materials, such as Sudlac and ReduHeat, have been developed to allow a high light level but reduce the amount of infrared light able to penetrate.

An alternative is woven or knitted shade fabrics including polypropylene, saran, polyethylene and polyester. These are lightweight, easy to apply and available in several degrees of shade from 10-90 percent. They usually have to be custom-fabricated to fit the greenhouse. Most materials are ultraviolet light stabilized and have a life of about 10 years. Cost depends on the level of shade but is usually between 10-40 cents per square foot. Polypropylene is strong, tough and highly resistant to flexing, abrasion and chemical attack. It shrinks about 1 percent when placed on the greenhouse. Saran is fireproof and shrinks about 3 percent, requiring that it be installed with a slight sag. Lock-stitched, knitted polyethylene netting does not fray or rip-run when cut. It is fire and mildew resistant. Metalized material, such as Aluminet, moderates both day and night temperatures.
Research by North Carolina State University agricultural engineer Dan Willits determined that the percentage shade ratings were accurate. The reduction in total energy was only half of the shade percentage due to conduction of heat from the material to the glazing. Black or green material, although it gives longer life, conducts more heat than white material. A 50 percent shade reduced radiation about 10°F. Keeping the material wet with spray nozzles will increase its effectiveness as it removes heat from the material.

Laboratory tests conclude that any shade material is more effective on a windy day as a greater amount of energy that is intercepted is removed before it is transferred to the interior. Tests also indicate any shade material performs better in reducing heat if the greenhouse interior is painted a light color.

Large Budget Solution
Interior mechanized screen systems are becoming popular for both gutter-connected and free-standing greenhouses. Usually referred to as energy/shade screens, these systems are eligible for USDA grant programs.

A shade screen is a mechanical system consisting of a drive motor, support cables, energy/shade material and controls. A single gear motor powers up to an acre shade system. The screen material is supported or suspended from the cables and can be closed or opened as needed. Control can be by timer, thermostat or light level sensor.

A retractable system has the advantage that it can be pulled back on cloudy days to allow maximum light transmission. Being located inside the greenhouse, the screen material is not affected by weather conditions.

A typical screen material is made of 4 mm wide aluminum and polyester strips held together with polyester filament yarn. The aluminum reflects the incoming sun back outside the greenhouse rather than it being absorbed. This reduces the inside temperature about 10°F. At night when the greenhouse is being heated, the screen is closed and it traps the heat within. Screen materials are light weight and available with a wide range of light transmission and energy conservation.

In free-standing greenhouses such as a hoophouse, the mechanical system to move a screen can be simple. Three or more light weight tracks with rollers are suspended from the frame. The rollers with clips support the screen material. This system can be opened and closed by hand or it can be fully automated. In a 100-foot greenhouse, the screen is usually split into two 50-foot sections that can be stored at each endwall.

Mechanized systems usually have 8- to 12-foot panels that move between the trusses or collar ties. The gear motor powers the drive cable or rack and pinion to move the panels. For energy conservation, an edge seal is needed to contain the heat under the screen. For shade purposes, this is not as important. Mechanized systems usually cost from $1-$4 per square foot installed.